1. Field of the Invention
The present invention is related to integrated circuit characterization methods and circuits, and more particularly to a circuit and method for measuring variations in device characteristics in a test integrated circuit.
2. Description of Related Art
Characterization integrated circuits are in common use to evaluate designs on processes and have become increasingly necessary as processes have shrunk. Ring oscillator circuits are frequently used in such designs to study device characteristic variations that lead to variations in circuit parameters such as delay time. Performance screen ring oscillators (PSROs) are implemented along with functional circuits in a production integrated circuit to assist in production screening and as a design/process improvement tool. Since it is relatively easy to accurately measure the output frequency of a ring oscillator, and the output frequency is typically directly related to delay time, frequency measurements provide a effective means for studying process and design effects on delay time.
However, ring oscillator test circuits typically do not provide information about random local variation of device characteristics, nor of the systematic and variational differences between the P-channel devices and N-channel devices. Since the frequency of oscillation of the ring oscillator is dependent on the aggregate delay around the ring, a ring oscillator test typically yields only oscillator-to-oscillator variation information, which may be die-to-die or among multiple ring oscillators on a die. There is no measurement of local variation within the ring oscillator itself.
Therefore, it would be desirable to provide a characterization method and ring oscillator circuit for determining device characteristic variation within the ring oscillator and between P-channel and N-channel devices within the ring oscillator, so that local random variation, systematic deviation and N-channel to P-channel skew can be measured.